| Contents |
| Overview |
| What Are Thunderstorms? |
| Children Of The Storm |
| Tornado Safety Rules |
| Lightning Safety Rules |
| First Aid |
Note: This is a combination of several weather safety files...including thunderstorms
and their elements...tornadoes, lightning, hail, and flash flooding.
U.S. Department of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
June 1988
Mechanical processes are also at work. Warm, buoyant air may be forced upward by the wedge-like undercutting of a cold air mass, or by flowing up a mountain slope. Winds blowing into the center of a low-pressure area may force warm air near the center upward.
In the first stage of thunderstorm development, an updraft carries warm air to a level where the air becomes saturated with moisture, and visible droplets appear as a cloud begins to form. Continued upward movement produces large clouds resembling rising mounds, domes, or towers, known as cumulus clouds. Strong winds above the developing clouds may further enhance the updraft or convection.
As the cloud forms, water vapor changes to liquid and/or frozen cloud particles. This results in a release of latent heat, that takes over as the principal source of energy for the developing cloud. Once the cloud starts to form by other forces, this release of heat helps keep it growing. The cloud particles grow by colliding and combining with each other, forming rain, snow, and hail (the process is known as coalescence. When the droplets become heavy enough to fall against the updraft, precipitation begins.
Having reached its final stage of growth, the cumulonimbus cloud, called a thunderstorm, may be several miles across its base, and often towers to altitudes of 40,000 feet or more. High level winds shred the cloud top into the familiar anvil form. These cloud towers are sometimes visible as lonely giants, and at other times while moving several abreast are known as a squall line.
This final stage is also marked by a change in wind flow within the storm cells. The updraft which initiated the cloud's growth no longer prevails, and is joined by a downdraft generated by the precipitation. This updraft-downdraft couplet constitutes a single storm ''cell''. Most storms are composed of several cells that form, survive for perhaps 20 minutes, and then die. New cells may replace old ones, and it's possible for some storms to last for several hours.
Strong gusts of cold wind from the downdraft, or heavy precipitation -- rain and hail -- often occur at the ground beneath and outward from the mature storm. Lightning always accompanies the thunderstorm. These are nature's warnings that the thunderstorm is in its most violent stage. Tornadoes may also be associated with the thunderstorm.
Even so, the thunderstorm cell has already begun to die. The violent downdraft, having shared the circulation with the sustaining updraft, now strangles it. Precipitation weakens, and the cold downdraft ceases. The thunderstorm cell, a short-lived creation spreads and dies.
During periods of heavy rain, carefully watch river levels, and be alert to upstream areas, where flash flooding can rapidly develop. Campers should avoid areas where heavy upstream rains may occur. In July, 1976, a 19 foot wall of water raced down the Big Thompson Canyon in Utah, the result of heavy thunderstorms upstream. Hundreds of campers were in the canyon that night, and 139 campers died in the resulting flash flood.
A FLASH FLOOD WATCH means that a Flash Flood threat exists in the indicated areas. Check preparedness requirements, keep informed, and be ready for quick action if flash flooding is observed, or a warning is issued.
A F LASH FLOOD WARNING means that flash flooding is occuring, or is imminent on
certain streams, or in certain localities. When the warning is given, get out of the danger
zone and to higher ground IMMEDIATELY!! Don't waste time trying to save personal
property. Be extremely cautious in crossing rain swollen streams, as rapidly rising water
may sweep you off your feet. Do not cross or drive into any area where the water is above
your knees, or of unknown depth. Most flash flood deaths are due to drowning. If your car
stalls, abandon it IMMEDIATELY, and run to higher ground. Act quickly! Your margin of
safety may be counted in seconds!!
A severe thunderstorm may spawn a tornado, a violently rotating column of air, which descends from a thunderstorm cloud system. The season of peak tornado occurence extends from March through September. But, the National Weather Service warns you that tornadoes can occur anytime, anywhere. However, they are most common in the Midwestern, Central, and Southern United States during the afternoon and evening hours.
Careful lookout should be kept during any period of severe thunderstorm activity for the
typical tornado shaped funnel, or the loud roar associated with them. The rapidly spinning
black or grey tornado funnel may range from a few yards to nearly a mile in width. It may
move slowly, or as fast as 90 mph; skipping along, or even making a U-turn. It may travel
along the ground a few hundred feet or several hundred miles; appearing singly, or in groups
of twisters. Winds in a tornado can reach speeds of 300 mph, and tornadoes can stay on the
ground from as little as 10 minutes to as long as several hours.
Sometimes, there is quite a bit of cold air aloft, and strong updrafts prevailing in thunderstorms. When this happens, hail may result. As the raindrop is carried up into the colder air at the top of the storm, it freezes, and falls from the weight of the ice...then melting. But, the updrafts will carry it back to the top of the storm to pick up another layer of ice, then falling again from the weight. The hailstone may make several roller coaster rides up and down in the storm, depending upon the strength of the updraft; until it becomes too heavy to be supported by the updraft, and the mature hailstone races to the Earth.
You can tell how many times a hailstone went through the storm by cutting it open, and
viewing the rings inside it. One of the largest hailstone ever recorded fell in Coffeyville,
Kansas on Sept. 3, 1970...weighing 1 2/3 pounds!! Also, if you're in an area where it's
hailing, tornadic development is likely near you, and you should seek protective shelter
immediately!!
The Earth is normally negatively charged with respect to the atmosphere. But as the thunderstorm passes over the ground, the negative charge in the base of the cloud induces a positive charge on the ground below, and for several miles around the storm. The ground charge follows the storm like an electrical shadow, growing stronger as the negative cloud charge increases. The attraction between positive and negative charges makes the positive ground current flow up buildings, trees, and other elevated objects in an effort to establish a flow of current. But air, which is a poor conductor of electricity, insulates the cloud and ground charges, preventing a flow of current until huge electrical charges are built up.
Lightning occurs when the difference between the positive and negative charges -- the electrical potential -- becomes great enough to overcome the resistance of the insulating air, and to force a conductive path for current to flow between the two charges. Electrical potential in these cases can be as much as 100 million volts. Lightning strokes proceed from cloud to cloud, cloud to ground, or, where high structures are involved, from ground to cloud.
2. Securely tied down mobile homes offer NO PROTECTIVE SHELTER in thunderstorms, or tornadoes, and should be abandoned for a more sturdy, permanent shelter. If in an automobile, ABANDON YOUR CAR!! DO NOT TRY TO OUTDRIVE THE TORNADO!!
3. The National Weather Service will broadcast Tornado Watches and Warnings over NOAA Weather Radio, between 162.40 and 162.55 megahertz on the public service band. If possible, purchase a unit that has a battery backup, and a warning alarm feature. A warning alarm tone precedes the issuance of any severe weather watch or warning; alerting you to possible danger.
4. A TORNADO WATCH means that conditions are favorable for tornadoes and severe thunderstorms in and close to the watch area. During the watch, continue your normal activities, but watch for rapidly changing weather conditions, and be ready to seek safe shelter on a moments notice. A TORNADO WARNING is an URGENT announcement that a tornado has been reported, and warns you to take immediate action to protect life and property. When warnings are issued...MOVE QUICKLY!! Seconds save lives!!
5. A SEVERE THUNDERSTORM WATCH is issued, when conditions are favored for the development of thunderstorms containing winds more than 57 mph, or hail 3/4 inch or more in diameter.
A SEVERE THUNDERSTORM WARNING is issued when severe thunderstorms are imminent, have been reported, or are indicated by Doppler Radar. Severe Thunderstorms can produce tornadoes AT ANY TIME.
2. When a thunderstorm threatens, get inside a home, a large building, or an all-metal (not convertible) automobile. Do not use the telephone except for emergencies.
3. If you are caught outside, do not stand underneath a tall isolated tree, or a telephone pole. Avoid projecting above the surrounding landscape. For example, don't stand on a hilltop. In a forest, seek shelter in a low area under a thick growth of small trees. In open areas, go to a low place, such as a ravine or valley.
4. Get off or away from open water, tractors, and other metal farm equipment, or small metal vehicles...such as motorcycles, bicycles, golf carts, etc. Put down golf clubs and take off golf shoes. Stay away from wire fences, clotheslines, metal pipes, and railroad tracks. If you are in a group in the open, spread out, keeping people several yards apart.
5. Remember -- lightning may strike at least 15 miles from the parent cloud...known as a BOLT FROM THE BLUE. Precautions should be taken even though the thunderstorm is not directly overhead. If you are caught in a level field or prairie, far from shelter...and if you feel your hair stand on end, lightning may be about to strike you. Drop to your knees, and bend forward, putting your hands on your knees. Do not lie flat on the ground. If lightning strikes near you in this position, the chances of it using your body as a conductor are minimized, but not eliminated.
The American Red Cross says that if a victim is not breathing, you should immediately begin mouth-to-mouth resuscitation, once every 5 seconds to adults and once every 3 seconds to infants and small children, until medical help arrives.
If both pulse and breathing are absent, cardiopulmonary resuscitation, or CPR -- a combination of mouth-to-mouth resuscitation and external cardiac compression -- is necessary. This procedure should be administered ONLY by persons with PROPER TRAINING... which can be obtained through your local American Red Cross chapter.
Victims who appear only stunned, or otherwise unhurt, may also need attention. Check for burns, especially at fingers and toes, and next to buckles and jewelry. Give first aid for shock. Do not let the victim walk around. Send someone for help, or call 911. Stay with the victim until help arrives. Be prepared.
A Red Cross first aid course provides excellent instruction on how to render aid to a person who has been struck by lightning.